Abstract: On June 11, 2012, Employee #1, of Vegas Tunnel Constructors, Las Vegas, NV, was part of a swing shift crew excavating a tunnel. They were using a tunnel boring machine (TBM). The TBM had been excavating in closed mode for approximately 17 days. Closed mode is a term used to describe one of two modes of operation this TBM was capable of performing, the other being open mode. Closed mode excavation involves the pressurization of the cutterhead and face of the TBM to support the ground and/or control excessive groundwater inflows. The position of the TBM found it excavating through a large vertical fault zone that later became the Detachment Fault. The ground conditions required the TBM face to be pressurized to approximately 13 bars of pressure. The grout being injected in the annular space was being pumped in at approximately 14 bars. The swing shift had begun work and was preparing to erect a ring segment set; Ring #135. The swing shift began to build Ring #135 at about 4:00 p.m. The first segment was successfully installed in the tunnel crown (top of the tunnel). The crew then began to install the second segment, which was to be placed in front of the keystone of Ring #134. Three hydraulic rams, including one abutting the keystone of Ring #134, were slowly retracted. Almost as soon as the retraction began, a loud pop was heard. Some members of the crew reported hearing a second pop. The keystone for Ring #134 had shifted forward approximately 4 inches. The come-a-long did not hold the forces exerted, and the gears were stripped out. This shift permitted grout and other materials such as rocks to escape the annulus at a pressure of approximately 14 bars (200 psi). Employee #1 was standing in front of the area where the gap developed. He was struck and killed by the moving grout and rocks. Coworkers found Employee #1 and removed him from the tunnel.